Need a picture to be sure but it sounds like you have whats called a fresh air heater in the 3800. They are desirable if the heater core is good. You also need heater valve to control the water flow/temp. My truck had a round Harrison heater with a motor. Sounds like what you might have in the 6100. Called a recirculating heater because it just warms the air already in the cab. Mine didn't have any defrost provisions. Some recirculating heaters do have defrost provisions. They usually have a Chevy/GMC badge. I think the majority, if not all, the dealer installed heaters were made by Harrison.
Chuck
